The present invention relates to an improved compound tool with a core of high speed steel and a cover of more wear resistant material.
U.S. Pat. No. 4,618,540, which is hereby incorporated by reference, discloses a compound tool for chip forming machining with superior properties consisting of a core of high speed steel and a surface of a hard material containing 30-70% by volume of particles selected from the group consisting of carbides, nitrides, carbonitrides and mixtures thereof of metals selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and mixtures thereof, in a matrix based on Fe, Ni and/or Co. The patent teaches that the high speed steel shall have a conventional composition, e.g., conventional HSS grades M2, M41, T2 and the like with, in particular, a carbon content that from experience and theory has been found to give optimal properties for that particular steel. By conventional composition is thus meant a composition found in international and national standards as well as in specifications from high speed steel manufacturers.
For example, a conventional high speed steel of M2-type has, according to literature data, a nominal composition of the alloying elements 0.3% Si, 0.3% Mn, 4.0% Cr, 5.0% Mo, 6.0% W and 2.0% V with a carbon content of 0.85-0.88 weight-%. To get a higher wear resistance, the carbon content can be increased and consequently about 1.0 weight-% C is a rather common level for M2. A certain variation of the above mentioned figures for the other alloying elements, of course, is allowed. According to the state of the art, the carbon content is considered as the optimum alloying element with regard to a balance in wear resistance and toughness. A decrease in carbon content increases the risk of precipitation of ferrite which gives soft zones in the material. Therefore, the carbon content is not usually considered to be lowered below the nominal amounts. A similar situation in regard to the risk associated with lower carbon content applies to other types of high speed steel.
High speed steel is commercially available in solid or powder state. It should be noted, however, that when a high speed steel powder contains oxygen, the carbon content is increased in order to compensate for the oxygen content. For example, a water granulated M2-powder general has an oxygen content of 0.2 weight-% and a carbon content of 1.1 weight-%. During sintering, carbon monoxide is formed reducing the carbon content in the final product to the convention value as set forth above.